(1) Field of the Invention
The present invention relates to a S-correction capacitor switching apparatus for a display monitor for switching S-correction capacitors coupled with a deflection coil depending on the frequency of a horizontal synchronizing signal of an input video signal.
(2) Description of the Prior Art
FIG. 1 is a circuit diagram showing the construction of a horizontal deflection circuit having a prior-art S-correction capacitor switching apparatus, disclosed in U.S. Pat. No. 4,709,193. In the Figure, the horizontal deflection circuit 1 includes a horizontal output transistor Q1, a resonance capacitor CR1, a damper diode D1, a horizontal deflection coil DY, a linearity coil LIN, a S-correction capacitor CS1, and a horizontal output transformer T1. Reference numeral 2 designates a part of a S-correction capacitor switching apparatus, reference symbols CS2 and CS3 designate S-correction capacitors to be switched, reference symbol Q2 designates a switching device for coupling the S-correction capacitor CS3 with the aforementioned S-correction capacitor CS1, and reference symbol Q3 designates a switching device for coupling the S-correction capacitor CS2 with the aforementioned S-correction capacitor CS1. The switching devices Q2 and Q3 each comprise a field effect transistor (FET). Reference symbol R1 designates a resistor coupled between the drain and source of the switching device Q3 and reference symbol R2 designates a resistor coupled between the drain and source of the switching device Q2. Reference symbol S1 designates a switching device control signal for controlling the conductive/nonconductive state of the switching device Q2 in accordance with the frequency of the horizontal synchronizing signal of an input video signal and reference symbol S2 designates a switching device control signal for controlling the conductive/nonconductive state of the switching device Q3 in accordance with the frequency of the horizontal synchronizing signal of an input video signal. Numeral 3 designates a variable power supply capable of changing voltage to be supplied to the horizontal deflection circuit 1 in accordance with horizontal frequency.
Operation of the circuit shown in FIG. 1 will now be described. When the frequency of the horizontal synchronizing signal of an input video signal changes, it is necessary to correct deterioration of the linearity of a display screen depending on the frequency of the horizontal synchronizing signal of an input video signal by changing the capacitance of the S-correction capacitor. Thus, the S-correction capacitors CS2 and CS3 are coupled in parallel with the S-correction capacitor CS1, which operates in wide horizontal frequency range of the horizontal deflection circuit 1, so as to correct the linearity of the displayed image corresponding to each predetermined horizontal frequency. In this case, the conductive/nonconductive state of the switching device Q3 coupled in series with the S-correction capacitor CS2 is controlled by the switching device controlling signal S2, and the conductive/nonconductive state of the switching device Q2 coupled in series with the S-correction capacitor CS3 is controlled by the switching device controlling signal S1 depending on the horizontal frequency, to correct the linearity of a display screen at each horizontal frequency.
Resistors R1 and R2 which are coupled between the drain and source of the aforementioned switching devices Q2 and Q3, respectively, are provided to prevent the switching devices Q2 and Q3 from being deteriorated due to an excessive voltage applied to the drains of the switching devices Q2 and Q3 because charged voltage occurs between the drain and source of each of the switching devices Q2 and Q3 when power is turned on under the condition that the switching devices Q2 and Q3 are in their nonconductive state or when the horizontal frequency changes. The resistors R1 and R2 discharge DC voltage components applied to the switching devices Q2 and Q3, thereby enabling use of the switching devices to be those having a low withstand voltage characteristic, and then realizing reduction of the cost of the apparatus.
Since the S-correction capacitor switching apparatus in the prior art display monitor has the above-described construction, it is possible to suppress, by discharging, the DC voltage components generated at the drains of the switching devices Q2 and Q3. However, because the oscillation frequency of the horizontal output circuit becomes unstable particularly when the power is turned on or off, there is a problem in that, if the capacitance of a selected S-correction capacitor is not suitable, an excessive voltage is applied to the drains of the switching device so that the FETs, which act as switches, may be damaged.